As disclosed e.g. in patent literature 1, there is known in the art an automotive seatback device used in a vehicle, in which left and right lateral frames are provided on a seatback frame, a planar elastic body for supporting a cushioning material is provided between the left and right lateral frames, a left side portion of the planar elastic body is connected to the left lateral frame with a plurality of left coil springs, a right side portion of the planar elastic body is connected to the right lateral frame with a plurality of right coil springs, and the planar elastic body is supported against the left and right lateral frames with the left and right coil springs.
Supporting the planar elastic body using the left and right coil springs deforms the left and right coil springs so that the planar elastic body can fit with the back of a vehicle occupant.
Normally, a head rest is provided at a top portion of the seat back. The head rest is positioned at a relative distance from the head of the vehicle occupant under normal driving conditions. As a result, it is difficult for the headrest to satisfactorily support the head of the vehicle occupant in an instance where another vehicle collides into a rear portion of the vehicle (i.e., a rear collision).
One way to address this problem is to support the planar elastic body so that the upper body of the vehicle occupant moves towards the rear of the vehicle body and the headrest supports the head, as with the seat back disclosed in patent literature 1. In other words, in an instance where another vehicle collides with the rear portion of the vehicle (i.e., a rear collision), the left and right coil springs stretch, the upper body of the vehicle occupant moves towards the rear of the vehicle body together with the planar elastic body, and the headrest supports the head.
However, according to the seatback device of patent literature 1, the left and right coil springs are provided so that an axial line of each of the coil springs is oriented in the direction of vehicle width. As a result, it is difficult to move the upper body of the vehicle occupant towards the rear of the vehicle body in a satisfactory manner, and it is difficult to adequately support the head of the vehicle occupant with the headrest.